1. Field of the Invention
The present invention relates to a display device, and more particularly to a display device which includes a thin film transistor in each pixel.
2. Related Art
This type of display device is configured such that a plurality of pixels is arranged on a display part in a matrix array, each pixel row formed of these pixels is sequentially selected by turning on the thin film transistors provided to the respective pixels in response to scanning signals supplied to the thin film transistors via the gate signal line, and video signals are supplied to the respective pixels of the pixel row via a drain signal line which is connected to corresponding pixels of another pixel row in common at timing of such selection.
With respect to the constitution of such a thin film transistor, there has been known the constitution in which the semiconductor layer, besides a region where the semiconductor is formed, extends to positions below a connecting portion between an electrode of the thin film transistor and the drain signal line respectively.
The thin film transistors having such constitutions can be formed through particular manufacturing steps, for example and, at the same time, due to such constitutions, it is possible to acquire an advantageous effect that the occurrence of a broken step of the drain signal line or the like can be obviated.
Display devices which include the thin film transistors having such constitutions are disclosed in patent document 1 (JP-A-2003-303973) or patent document 2 (JP-A-2005-303119), for example.
However, the display device having the above-mentioned constitution is configured to have a pattern, as viewed in a plan view, in which the semiconductor layer has a portion which projects outwardly from a profile portion defined by a drain electrode and a source electrode of the thin film transistor, a channel region between the drain electrode and the source electrode, the drain signal line, and the connecting portion between the drain signal line and the drain electrode.
Accordingly, for example, there arises a phenomenon that a charge (a carrier) which is generated in the semiconductor layer below the drain signal line due to the radiation of light (light from a back light, for example) to the semiconductor layer below the drain signal line passes the protruding portion of the semiconductor layer and flows into a channel region of the thin film transistor.
That is, the thin film transistor is configured to easily generate leaking of light, and this leaking of light brings about lowering of display characteristics such as crosstalk, brightness irregularities and lowering of contrast.
These drawbacks have been also pointed out by the above-mentioned patent document 1 and patent document 2. To overcome such drawbacks, the technique disclosed in patent document 1 adopts a method which changes a pattern of the semiconductor layer formed by a photolithography technique, and the technique disclosed in patent document 2 adopts a method which makes a surface of the protruding portion of the semiconductor layer coarse.
However, neither patent document 1 nor patent document 2 discloses the formation of the thin film transistor using a registry flow method, the formation of drain electrodes in a particular shape or the like.